Fluid controller and logic control system for use therewith

ABSTRACT

An improved fluid controller (15) is provided having both rotary valving (47) actuated by the steering wheel (17), and axial valving (49) controlled by an electro-hydraulic valve (113) in response to a correction signal (31). In one embodiment, both the rotary valving and the axial valving are achieved by a primary valve member (63) and a follow-up valve member (65), and both the rotary and axial valving can be actuated simultaneously. Also disclosed is a logic control system for closed-loop control of the electro-hydraulic valve, whereby performance problems such as self-steering, wander, drift, and others are substantially eliminated.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a Divisional application of co-pending applicationU.S. Ser. No. 703,318, filed May 20, 1991, now U.S. Pat. No. 5,115,640,which is a Continuation-in-Part of co-pending application U.S. Ser. No.513,366, filed Apr. 23, 1990, in the name of Dwight B. Stephenson for a"STEERING VALVE WITH INTEGRAL PARALLEL CONTROL", now U.S. Pat. No.5,016,672.

BACKGROUND OF THE INVENTION

The present invention relates to fluid controllers of the type used tocontrol the flow of fluid from a source of pressurized fluid to a fluidpressure actuated device such as a steering cylinder which comprisespart of a full-fluid-linked steering system. More particularly, thepresent invention relates to such a steering system including anauxiliary, parallel fluid path and a logic control system forcontrolling such parallel fluid path.

Although the present invention may be used in connection with fluidcontrollers of many types, and having various applications, it isespecially advantageous when used with a fluid controller of a type usedin full-fluid-linked steering systems, and will be described inconnection therewith.

A typical fluid controller of the type to which the present inventionrelates includes a housing which defines various fluid ports, andfurther includes a fluid meter, a valve means, and an arrangement forimparting follow-up movement to the valve means, in response to the flowof fluid through the fluid meter. The flow through the controller valvemeans is directly proportional to the area of the variable flow controlorifices in the main fluid path, the area of the flow control orificesin turn being proportional to the rate at which the steering wheel isrotated.

A typical example of a vehicle which utilizes a fluid controller of thetype to which this invention relates would be an agricultural tractor orcombine. There is growing interest in being able to steer such vehiclesby means of an electro-hydraulic steering system, and preferably, onewhich is "closed loop", i.e., one in which there is continuouscorrection of any "error" between the position of the steered wheels andthe position of the steering wheel.

Prior to the present invention, in order to provide a vehicle with bothconventional rotary input, full-fluid-linked steering, and some sort ofclosed loop, electro-hydraulic control, would have required that thevehicle have both a conventional fluid controller and a separate,parallel control system, operable in response to various signals, suchas a steered wheel position signal and a steering wheel position signal.Although such systems have been generally known, at least in concept,there has been very limited commercial use of such systems.

One of the problems associated with such systems is the difficulty ofcoordinating operation of the conventional fluid controller with that ofthe parallel, electro-hydraulic valve. For example, under certaincircumstances, it is desirable for the fluid controller to override theelectro-hydraulic valve. It is also desirable, and on many vehicleapplications it is absolutely necessary, to be able to manually steerthe vehicle with the fluid controller, thus further complicating thecoordination between the fluid controller and the electro-hydraulicvalve.

Another problem area associated with systems including both fluidcontrollers and parallel electro-hydraulic valves relates to a series ofsteering performance criteria which are of particular concern to thevehicle operator. These performance criteria are of special concern insuch systems where a portion of the flow to the steering cylinder isindependent of the fluid controller and the fluid meter which istypically included in such controllers. For example, there should be no"wander" or "drift" in the steering system, i.e., the steered wheelsshould not move whenever the operator is not rotating the steeringwheel. As another example, there should be proper "knob" control, i.e.,whenever the vehicle operator returns a steering wheel knob to a certainposition, the steered wheels should always return to a correspondingposition. As a final example, it is desirable that corrections made bymeans of the parallel, electro-hydraulic valve not be readily apparentto the vehicle operator, such that the vehicle operator has theperception of not being fully in control of the steering system.

SUMMARY OF THE INVENTION

Accordingly, it is an object of the present invention to provide animproved fluid controller and a parallel electro-hydraulic control valvewithout the need for separate, expensive valves, external to the fluidcontroller, while maintaining the capability of manually steering, orhaving the fluid controller override the operation of theelectro-hydraulic valve, especially in the event of a malfunction in theelectronic controls.

It is a more specific object of the present invention to provide animproved fluid controller which achieves the above-stated object byproviding the parallel, electro-hydraulic valve within the fluidcontroller.

It is another object of the present invention to provide an improvedfluid controller and logic control system for controlling the parallel,electro-hydraulic valve which satisfies various performance criteriasuch as those set forth above.

The above and other objects of the present invention are accomplished bythe provision of an improved fluid controller operable to control theflow of fluid from a source of pressurized fluid to a fluid pressureoperated device. The controller is of the type including housing meansdefining an inlet port for connection to the source of pressurizedfluid, a return port for connection with the system reservoir, and firstand second control fluid ports for connection to the fluid pressureoperated device. Valve means is disposed in the housing means, andcomprises a primary, rotatable valve member and a cooperating,relatively rotatable follow-up valve member, the primary and follow-upvalve members defining a neutral rotary position and a rotary operatingposition in which the primary valve member is rotatably displaced fromthe neutral rotary position relative to the follow-up valve member. Thehousing means and the valve members cooperate to define a main fluidpath providing fluid communication from the inlet port to the firstcontrol fluid port, and from the second control fluid port to the returnport, when the valve members are in the rotary operating position. Theprimary and follow-up valve members define a neutral axial position andan axial operating position. The controller includes means operable tobias the valve members toward the neutral axial position and meansoperable to displace the valve members to the axial operating position.

The primary valve member defines first and second axial fluid passages,and the follow-up valve member defines a first axial fluid port incontinuous fluid communication with the inlet port, and a second axialfluid port in continuous fluid communication with the first controlfluid port. The first and second axial fluid ports are blocked fromfluid communication with the first and second axial fluid passages,respectively, where the valve members are in the neutral axial position.The first and second axial fluid ports are in fluid communication withthe first and second axial fluid passages, respectively, when the valvemembers are in the axial operating position to thereby define a portionof a parallel fluid path. The axial fluid ports and the axial fluidpassages are configured such that when the primary and follow-up valvemembers are simultaneously defining the rotary operating position andthe axial operating position, the valve members and the housing meanscooperate to define the main fluid path and the parallel fluid path,simultaneously, whereby the total flow to the fluid pressure operateddevice is approximately the sum of the flows in the main and parallelfluid paths.

In accordance with another aspect of the present invention, there isprovided an improved method of controlling the flow of fluid from asource of pressurized fluid through a fluid controller in response tothe position and movement of an input device, to cause the position of asteering cylinder to conform to the position of the input device. Themethod comprises the steps of providing the fluid controller with mainvalving operable to define a main fluid path, and control fluid flowtherethrough, in response to the movement of the input device, andauxiliary valving operable to define an auxiliary fluid path, andcontrol the flow of fluid therethrough, in response to changes in acommand signal. The method further comprises the steps of sensing theposition of the input device and generating an input position signal,sensing the position of the steering cylinder and generating an outputposition signal, and comparing the output position signal to the inputposition signal and generating the command signal. The method includesthe final step of transmitting a signal representative of the commandsignal to the auxiliary valving and modulating the auxiliary valving todrive the output position signal toward the input position signal.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view of a hydrostatic power steering system madein accordance with the present invention.

FIG. 2 is an axial cross-section of the fluid controller shownschematically in FIG. 1.

FIG. 3 is an enlarged, fragmentary, axial cross-section, similar to FIG.2, illustrating one detailed aspect of the present invention.

FIG. 4 is a transverse cross-section taken on line 4--4 of FIG. 3, andon approximately the same scale.

FIG. 5 is an enlarged, fragmentary, axial cross-section, similar to FIG.2, illustrating another detailed aspect of the present invention.

FIG. 6 is an elevation view of the primary valve member of the fluidcontroller shown in FIG. 2.

FIG. 7 is an elevation view of the follow-up valve member of the fluidcontroller shown in FIG. 2, and on substantially the same scale as FIG.6, with a portion of the valve housing (in axial cross-section)included.

FIG. 8 is an overlay view of the valving used in the fluid controllershown in FIG. 2, but on a larger scale than in FIG. 2, and with thevalving in the rotary neutral and axial neutral position.

FIG. 9 is an enlarged, fragmentary, overlay view, similar to FIG. 8, butwith the valving in a rotary operating, neutral axial, position.

FIG. 10 is a further enlarged, fragmentary, overlay view, similar toFIG. 9, but with the valving in both a rotary operating and axialoperating position.

FIG. 11 is an enlarged, fragmentary, overlay view, similar to FIG. 10,and on the same scale, with the valving in a maximum rotary operatingposition.

FIG. 12 is a block diagram of a control logic system in which the fluidcontroller of the present invention may be utilized.

FIG. 13 is a superimposed graph of steering wheel position (in degrees)and steered wheel position (in degrees) versus absolute steering wheeldisplacement.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring now to the drawings, which are not intended to limit theinvention, FIG. 1 illustrates a vehicle hydrostatic power steeringsystem, including a fluid controller made in accordance with the presentinvention. The system includes a fluid pump 11, shown herein as a flowand pressure compensated pump, having its inlet connected to a systemreservoir 13. The system includes a fluid controller, generallydesignated 15, which receives rotary input by means of a steering wheel17, and controls the flow of fluid from the pump 11 to a fluid pressureoperated vehicle steering cylinder 19.

In accordance with one important aspect of the present invention, thesteering system shown in FIG. 1 includes a steering wheel positionsensor 21, which generates an input position signal 23, indicative ofthe instantaneous rotational position of the steering wheel 17. Thesystem also includes a steered wheel position sensor 25 (whichpreferably is physically associated with the cylinder 19), whichgenerates an output position signal 27. It will be understood by thoseskilled in the art that references hereinafter to "steered wheelposition" are actually referring to the position of the steeringcylinder 19 which, because of its mechanical connection to the steeredwheels, is representative of the position thereof.

The input and output position signals 23 and 27 are transmitted to alogic device which, in the subject embodiment, will be described, by wayof example only, as a rate select switch and controller 29, the functionof which will be described in greater detail subsequently. As is wellknown to those skilled in the art, the switch 29 compares the signals 23and 27, and generates a correction signal 31, which is representative ofthe difference between the desired position of the steered wheels (asindicated by the signal 23) and the actual position of the steeredwheels (as indicated by the signal 27). The correction signal is on ascale representative of the rate selected by the vehicle operator, forexample, either a "HIGH" rate or a "LOW" rate. The correction signal 31is transmitted to a pilot stage control, generally designated 33, whichwill be described in greater detail subsequently.

Referring still to FIG. 1, the fluid controller 15 includes an inletport 35, a return port 37, and a pair of control (cylinder) fluid ports39 and 41, which are connected to the opposite ends of the steeringcylinder 19. The fluid controller 15 further includes a load signal port43 which is connected to the flow and pressure compensator of the pump11 by means of a signal line 45.

In accordance with a primary feature of the present invention, the fluidcontroller 15 includes valving which is able to perform two distinct,but related, functions, as was illustrated and described in co-pendingapplication U.S. Ser. No. 513,366, now U.S. Pat. No. 5,016,672, which isincorporated herein by reference. The valving includes rotary valving,generally designated 47, the function of which is to define a main fluidpath to the cylinder 19, and axial valving, generally designated 49, thefunction of which is to define a parallel fluid path to the steeringcylinder 19. Actuation of the rotary valving 47 is controlled by thesteering wheel 17, while actuation of the axial valving 49 is controlledby the pilot stage 33, but may be overridden by rotation of the steeringwheel 17, as will be described in greater detail subsequently.

The valving arrangement which has been described generally above isillustrated and described in greater detail in above-incorporated U.S.Ser. No. 513,366, now U.S. Pat. No. 5,016,672. The fluid controller ofthe parent application may have either rotary input, to define the mainfluid path, or axial input to define the parallel fluid path, but doesnot illustrate or describe a valve which would function to define boththe main and parallel fluid paths, simultaneously.

In accordance with one aspect of the present invention, the valving ofthe fluid controller is configured to have both rotary and axial inputs,simultaneously. By way of general explanation and example only, but notlimitation, the rotary input actuates the valving to select direction ofsteering and direct a certain amount of fluid through the main fluidpath, while the axial input actuates the valving to select the remainderof the fluid required for precise control of the total amount of fluidrequired by the steering cylinder 19. Alternatively, and again by way ofexample only, the axial input can be used to provide eithermultiple-ratio or variable-ratio steering.

Referring now to FIG. 2, the fluid controller 15 comprises severalsections, including a valve housing 51, a wear plate 53, a sectioncomprising a fluid meter 55 (see also FIG. 1), and an endcap 57. Thesesections are held together in tight sealing engagement by means of aplurality of bolts 59 (only one of which is shown in FIG. 2), which arein threaded engagement with the valve housing 51.

The valve housing 51 defines the inlet port 35, the return port 37, thecontrol ports 39 and 41, and the load sensing port 43. The valve housing51 further defines a valve bore 61, and rotatably disposed therein is avalving arrangement comprising a primary, rotatable valve member 63(referred to hereinafter as the "spool"), and a cooperating, relativelyrotatable follow-up valve member 65 (referred to hereinafter as the"sleeve"). At the forward end of the spool 63 is a portion having areduced diameter, and defining a set of internal splines 67, whichprovide for a direct mechanical connection between the spool 63 and thesteering wheel 17. The spool 63 and sleeve 65 will be described ingreater detail subsequently, but it should be noted that the spool andsleeve together define both the rotary valving 47 and the axial valving49.

The fluid meter 55 may be of the type well known in the art, and in thesubject embodiment, includes an internally-toothed ring member 69, andan externally-toothed star member 71, which is eccentrically disposedwithin the ring 69 for orbital and rotational movement relative thereto.The star 71 defines a set of internal splines 73, and in splinedengagement therewith is a set of external splines 75 formed at therearward end of a drive shaft 77. The drive shaft 77 has a bifurcatedforward end 79 permitting driving connection between the shaft 77 andthe sleeve 65, by means of a drive pin 81. The ends of the pin 81 passthrough a pair of oversized pin openings 83 (see FIG. 8), defined by thespool 63, and are received in relatively close-fitting openings in thesleeve 65. The configuration of the pin openings 83 will be described ingreater detail in connection with the spool-sleeve layouts of FIGS. 8through 11.

As is well known to those skilled in the art, pressurized fluid flowingfrom the inlet port 35 through the various passages defined by the spool63 and sleeve 65 then flows through the fluid meter 55, causing orbitaland rotational movement of the star 71 within the ring 69. Such movementof the star 71 causes rotational follow-up movement of the sleeve 65, bymeans of the drive shaft 77 and drive pin 81, to maintain a particularrelative rotational displacement (referred to hereinafter as a "rotaryoperating position") between the spool 63 and sleeve 65, proportional tothe rate of rotation to the steering wheel 17. Disposed adjacent theforward end (left end in FIG. 2) of the spool 63 is a neutral centeringspring arrangement, generally designated 85, of the type which isillustrated and described in greater detail in co-pending applicationU.S. Ser. No. 602,829, filed Oct. 29, 1990, now U.S. Pat. No. 5,080,135,in the name of Dwight B. Stephenson, for a "LARGE DEFLECTION ANGLEROTARY MODULATION STEERING VALVE", assigned to the assignee of thepresent invention and incorporated herein by reference. Typically, thecentering spring 85 would include at least one helical, coiledcompression spring biasing the sleeve 65 toward a "rotary neutral"position (as that term will be defined in connection with FIG. 8),relative to the spool 63.

Referring still to FIG. 2, the valve bore 61 of the valve housing 51defines a plurality of annular fluid chambers surrounding the sleeve 65,to provide fluid communication between the various ports and the outersurface of the sleeve 65. An annular chamber 35c receives pressurizedfluid from the inlet port 35, while an annular chamber 37c communicatesreturn fluid to the return port 37. An annular chamber 39c providescommunication to or from the control port 39, while an annular chamber41c provides communication to or from the control port 41. Finally, anannular chamber 43c provides communication with the load signal port 43.

The toothed interaction of the star 71, orbiting within the ring 69,defines a plurality of expanding and contracting fluid volume chambers87, and adjacent each such chamber 87, the port plate 53 defines a fluidport 89. The valve housing 51 defines a plurality of axial bores 91(only one of which is shown in FIG. 2), each of which is in opencommunication with one of the fluid ports 89. The valve housing 51further defines a pair of radial bores 93L and 93R, providingcommunication between each of the axial bores 91 and the valve bore 61,as will be described in greater detail subsequently.

It is believed that the normal rotary action of a controller of thegeneral type shown in FIG. 1 is now well known to those skilled in theart, and such operation will be described only briefly herein. As thesteering wheel 17 is rotated, for example, in the clockwise direction,the spool 63 is also rotated clockwise, as viewed by the vehicleoperator, opening up a series of variable flow control orifices(comprising the rotary valving 47) between the spool 63 and the sleeve65. These orifices permit fluid communication from the inlet port 35through several of the orifices in series, and then through the radialbores 93R and the axial bores 91 to the expanding volume chambers 87 ofthe fluid meter 55. Fluid flowing from the contracting volume chambersof the fluid meter flows through the other of the axial bores 91, thenthrough the radial bores 93L, then through another variable orifice inthe valving and out the cylinder port 39 to the steering cylinder 19.Fluid returning from the cylinder enters the cylinder port 41, thenflows through another variable orifice in the valving, and then out tothe return port 37. The above-described fluid path is the one which hasbeen described herein as the "main fluid path", and the use of that termhereinafter will be understood to mean the above-described fluid path,or some portion thereof, when the spool and sleeve are in the rotaryoperating position.

It should be noted that all of the elements described up to this pointare element which are already known, and have been illustrated anddescribed in various prior art patents assigned to the assignee of thepresent invention. The newly added elements which comprise the variousaspects of the present invention will now be described, although many ofthe elements to be described are illustrated and described in theabove-incorporated parent application. In conventional controllers ofthe spool-sleeve type, the area of the flow orifices in the valving ischanged in response only to relative rotation between the spool and thesleeve. Thus, in such controllers, it has been typical for the axiallength of the sleeve to be substantially the same as that of the spool(excluding the reduced diameter portion).

It is one important aspect of the present invention to be able to defineflow control orifices in the valving in response to both relative rotarymotion of the spool and sleeve, and relative axial motion of the spooland sleeve, and as was mentioned previously, to do so simultaneously. Inthe subject embodiment, by way of example and not limitation, suchrelative axial motion is accomplished by making the sleeve 65 axiallyshorter than the adjacent portion of the spool 63, and providing meansfor axially displacing the sleeve 65, relative to the spool 63, from aneutral axial position to "an axial operating position", as that termwill be described in greater detail subsequently.

Referring now primarily to FIGS. 3 and 4, disposed radially between thevalve housing 51 and the spool 63 is a seal gland 95, held in placerelative to the valve housing 51 by means of a snap ring 97. As may bestbe seen in FIG. 4, the seal gland 95 includes four arcuate portions 99,which extend axially rearward (to the right in FIG. 3) from the mainbody of the seal gland 95. The arcuate portions 99 are closely spacedapart from an adjacent cylindrical surface defined by the valve housing51. Disposed radially inwardly from the arcuate portions 99 is a ballbearing set 101. The arcuate portions 99 cooperate with the valvehousing 51 and the ball bearing set 101 to define four generally squareopenings 102, and disposed within each of these square openings 102 is acoiled compression spring 103, the springs 103 being seated against themain body of the seal glands 95 at their left end, and against a wearwasher 105 at the right end.

Referring now to FIG. 5, there is illustrated an arrangement similar tothat shown in FIG. 3, wherein the port plate 53 and sleeve 65 cooperateto define a chamber 106, and the port plate 53 defines four openings,and disposed within each opening is a coiled compression spring 107.Each of the springs 107 has its right end in FIG. 5 seated against theport plate 53, and its left end seated against a wear washer 109. Theprimary function of the springs 103 and the springs 107 is to bias thesleeve 65 toward the neutral axial position shown in FIGS. 3 and 5 or,subsequent to displacement of the sleeve 65 to an axial operatingposition, to return the sleeve 65 to the neutral axial position.

Referring again to FIGS. 3 and 5, the chamber 102, when pressurized,biases the sleeve 65 to the right in FIG. 3, corresponding generally tothe axial operating position illustrated in FIGS. 10 and 11, which willbe described subsequently. With the chamber 102 pressurized, thecompression springs 107 in the chamber 106 (see FIG. 5) are compressed.Similarly, the chamber 106, when pressurized, biases the sleeve 65 tothe left in FIG. 5. With the chamber 106 pressurized, the compressionsprings 103 in the openings 102 are compressed.

Referring again primarily to FIG. 2, an electro-hydraulic controlcircuit for effecting axial actuation of the sleeve 65 will be describedbriefly. The endcap 57 defines a stepped bore 111, adapted to receive anelectromagnetic solenoid valve, generally designated 113, whichcomprises the pilot stage control 33 shown schematically in FIG. 1. Thesolenoid valve 113 includes a valve spool 115, which is movable from aneutral, centered position as shown in FIG. 2, to either an upwardactuated position, or a downward actuated position. The construction andfunction of the solenoid valve 113 is illustrated and described ingreater detail in the above-incorporated parent application, and will bedescribed only briefly herein.

As is illustrated somewhat schematically in FIG. 2, the endcap 57defines a fluid passage 117 in communication with the chamber 102.Similarly, the endcap 57 defines a fluid passage 119 in communicationwith the chamber 106, by means of a reduced diameter portion of the bolt59. The endcap 57 further defines a port 121, which may be incommunication with the system reservoir 13, or with some other source ofpressure at either a low pressure or a high pressure. Assuming forpurposes of explanation that the port 121 is connected to the reservoir,when the valve spool 115 is in the centered position shown in FIG. 2,both of the chambers 102 and 106 are ported to tank and the sleeve 65remains in the axial neutral position shown in FIGS. 2, 7, and 9. Adescription of the actuation of the solenoid valve 113 to shift thesleeve 65 to an axial operating position requires a more detaileddescription of the spool and sleeve, and therefore, will be providedsubsequently. Within the scope of the present invention, the solenoidvalve 113 could be of the ON/OFF type, in which case it could bepulse-width-modulated between the centered position shown in FIG. 2 anda desired actuated position. Alternatively, the solenoid valve 113 couldbe of the type providing proportional control anywhere between the twoextreme actuated positions. Various other electro-hydraulic controlarrangements could also be utilized, all of which are intended to bewithin the scope of the invention, as long as they perform the functionset forth in the appended claims.

VALVING ARRANGEMENT

Referring now to FIGS. 6 through 8, the spool 63 and sleeve 65 will bedescribed in greater detail, with regard to the various ports andpassages defined by the spool and sleeve. In connection with thesubsequent description, it should be noted that certain of the ports andpassages are arranged symmetrically, or generally symmetrically, withrespect to an imaginary central reference plain RP (see FIG. 8), andsuch elements will be described by reference numerals followed by eitheran "L" or an "R", to indicate that the element is located on either theleft side or the right side, respectively, of the reference plain RP. Onthe other hand certain of the elements do not have a correspondingelement, oppositely disposed about the reference plain RP, and will bereferred to by use of a reference numeral alone.

It should be understood that the overlay views of FIGS. 8 through 11 areintended to illustrate primarily the interface between the spool 63(dashed lines) and the sleeve 65 (solid lines), and as a result, certainstructural features which appear only on the outer surface of the sleeve65 will be shown in the elevation view of FIG. 7, but may not berepeated in all of the subsequent spool-sleeve overlay views. It shouldbe noted that in FIGS. 6 and 7, the spool 63 and sleeve 65 are in arelative axial position corresponding to the neutral axial positionshown in FIGS. 2, 8, and 9.

The spool 63 defines a pair of annular meter grooves 123L and 123R,which are axially aligned with the radial bores 93L and 93R,respectively. In communication with the meter groove 125L is a pressurepassage 125L, and in communication with the meter groove 123R is apressure passage 125R. Circumferentially adjacent each of the pressurepassages 125L is a return passage 127L, and circumferentially adjacenteach of the pressure passages 125R, but in the opposite direction, is areturn passage 127R. Toward the left end in FIGS. 6 and 8, the spool 63defines four tank ports 129. Both the return passages 127L and 127R andthe tank ports 129 extend to the interior of the spool 63.

Referring now to FIGS. 7 and 8, the sleeve 65 defines two pairs ofpressure ports 131, disposed somewhat to the right of reference planeRP. Disposed to the left of each pair of pressure ports 131 is a pair ofload sensing ports 133, both of which are in open communication with anannular load sensing groove 135, defined on the outer surface of thesleeve 65. The groove 135 is in continuous communication with the loadsensing port 43 by means of the annular chamber 43c. The sleeve 65further defines an operating port 137L disposed adjacent the returnpassage 127L, and an operating port 137R, disposed adjacent the returnpassage 127R. The sleeve 65 further defines a plurality of meter ports139L in continuous communication with the meter groove 123L, and aplurality of meter ports 139R, in continuous communication with themeter groove 123R. Each of the meter ports 139L and 139R includes agenerally rectangular opening defined by the outer surface of thesleeve, and shown only in FIGS. 7 and 8, the purpose of the rectangularopenings being to permit commutating fluid communication between themeter ports 139L and 139R, and the radial bores 93L and 93R,respectively, even when the sleeve 65 has been axially displaced in onedirection or the other from the neutral axial position shown in FIGS. 7and 8.

Finally, the sleeve 65 defines a plurality of tank ports 141, each ofwhich is in continuous communication with the return port 37 by means ofthe annular chamber 37c. All of the ports and passages described thusfar relate to the rotary valving 47 which, because it is generally wellknown in the art, will be described only briefly hereinafter.

Referring now to FIG. 9, if the steering wheel 17 and the spool 63 arerotated clockwise, (the spool 63 moves "down" in FIG. 9), pressurizedfluid flows from the inlet port 35 to the annular chamber 35c, thenthrough the pressure ports 131, which are now overlapping the pressurepassage 125R to define a main variable flow control orifice (A1r), thedesignation "r" after "A1" merely indicating an orifice formed inresponse to relative rotational movement of the spool and sleeve (seealso FIG. 1). At the same time, the pressure in the passage 125R(downstream of the A1r orifice) is being "sensed" or communicatedthrough the upper load sensing port 133 to the load sensing port 43 inthe manner described previously. Fluid flowing through the A1r orificethen flows into the meter groove 123R, then through the meter ports 139Rto the fluid meter 55, returning from the meter and flowing through themeter ports 139L and into the meter groove 123L. This "metered fluid"then enters the pressure passages 125L, which is now overlapping theoperating ports 137L to define a variable flow control orifice (A4r).Metered fluid flows from the A4r orifice into the annular chamber 39c,and from there to the control port 39, then to the steering cylinder 19.Fluid returning from the exhaust side of the steering cylinder 19 flowsto the control port 41, then into the annular chamber 41c, and thenthrough an A5r orifice defined by the overlap of the operating ports137R and the return passages 127R. This return fluid then flows throughthe interior of the spool 63, then radially out through the tank ports129, and the tank ports 141 to the annular chamber 37c, and then to thereturn port 37, and then to the system reservoir 13. The above-describedflow path thus comprises the "main fluid path" defined when the spooland sleeve are displaced from the neutral rotary position to a rotaryoperating position. However, it should be noted that in FIGS. 8 and 9,the spool and sleeve are both still in a neutral axial position relativeto each other.

AXIAL ACTUATION OF VALVING

As was the case in the above-incorporated parent application, it is animportant aspect of the present invention that the valve members be laidout such that axial actuation thereof results in a parallel fluid pathbeing defined within the fluid controller, which is preferably separateand distinct from the main fluid path which is defined in response torelative rotation of the spool and sleeve. As used herein, "separate anddistinct" in reference to the main and parallel fluid paths refersprimarily to the fact that the main fluid path flows through the fluidmeter 55, whereas the parallel fluid path does not. Obviously, the mainand parallel fluid paths are not totally separate and distinct from thestandpoint that both paths commence in the annular chamber 35c, and theseparate paths recombine at the annular chamber 39c (in the case of aright turn).

In connection with the subsequent description of the axial valving 49,and in the appended claims, many elements (such as ports, passages,etc.) are referred to by means of the term "axial". It will beunderstood by those skilled in the art that such use of the term "axial"is not necessarily intended to denote a structural feature of theparticular element, or a particular orientation, but instead, isintended to indicate that the particular element is related to the axialactuation of the sleeve 65, or is involved in the axial valving 49.

Referring first to FIGS. 8 and 9, it will be noted that substantiallyall of the elements described so far relate to the rotary valving 47,and that many of those elements are axially adjacent the pin openings83. It may also be seen in FIGS. 8 and 9 that, disposedcircumferentially between the pin openings 83 is an area of ports andpassages, not yet described in detail, which comprises the axial valving49.

Referring now primarily to FIG. 10, the axial valving 49 will bedescribed in some detail. The spool 63 defines a plurality of pressurepassages 151L, which are interconnected by a circumferentially extendingpassage 153L, and the spool 63 further defines a plurality of pressurepassages 151R, which are interconnected by a circumferentially extendingpassage 153R. The pressure passages 151L are in unrestrictedcommunication with a plurality of operating passages 155L (which aredisposed toward the right in FIG. 10) by means of an axial connectionpassage 157. Similarly, the pressure passages 151R are in unrestrictedcommunication with a plurality of operating passages 155R (which aredisposed toward the left in FIG. 10), by the axial connection passage157. The spool 63 further defines a plurality of tank passages 159L anda plurality of tank passages 159R. In describing the axial valving 49,there are three of each of the various passages which were described bythe phrase "a plurality of", but as will be understood by those skilledin the art, that number is not a significant aspect of the invention,but instead, has to do with providing the desired flow capacity throughthe parallel fluid path.

The sleeve 65 defines a plurality of pressure ports 161 which are incontinuous communication with the inlet port 135 through the annularchamber 35c. Disposed to the left of the pressure ports 161 is a singleload sensing port 163. The sleeve 65 also defines a plurality ofoperating ports 165L (disposed toward the right end in FIG. 10), and aplurality of operating ports 165R (disposed toward the left end in FIG.10).

With the steering wheel 17 rotated clockwise for a right turn, the spool63 is rotated, relative to the sleeve 65, to the rotary operatingposition already described in connection with FIG. 9. In accordance withone important aspect of the present invention, the rotary valving 47 isintentionally designed such that any particular rotation of the steeringwheel 17 will result in a flow through the main fluid path (the rotaryvalving 47) which results in a displacement of the steering cylinder 19which is less than that selected by the steering wheel 17. In otherwords, for any given rotation of the steering wheel 17, a comparison ofthe input position signal 23 and the output position signal 27 willalways result in an "error", thus resulting in the generation signal 31indicating that additional flow is required through the axial valving49, to achieve the desired position of the steering cylinder 19.

Therefore, during normal steering operations, there is always acorrection signal 31 transmitted to the pilot stage control 33 of FIG. 1to adjust the flow through the axial valving 49. Referring again to FIG.2, the correction signal 31 is transmitted to the solenoid valve 113which, assuming a right turn condition exists, results in downwardmovement of the valve spool 115 to drain the forward chamber 102 throughthe passage 117, while blocking drainage of the chamber 106 through thepassage 119. As a result, pressure builds in the chamber 106, and thesleeve 65 shifts to the left in FIG. 2 to the position shown in FIG. 10.Typically the chamber 106 would be pressurized by restricting the linedownstream of the return port 37, thus causing pressure to build withinthe spool 63, and communicating that pressure into the chamber 106through a passage 108. Similarly, the chamber 102 may be pressurizedthrough a passage 104.

Subsequently, the correction signal 31 will vary, thus varying theposition of the spool 115 and the axial position of the spool 65, tojust maintain a flow through the axial valving 49 which, when added tothe flow through the rotary valving 47, results in the desired positionof the steering cylinder 19.

Referring again to FIG. 10, with the sleeve 65 shifted to the left, tothe position shown, each of the pressure ports 161 is in communicationwith one of the pressure passages 151R, the cumulative overlaptherebetween defining an A1a orifice, the designation "a" after the "A1"merely indicating an orifice formed in response to relative axial motionof the spool and sleeve. Pressurized fluid entering the pressurepassages 151R then flows through the axial connection passage 157,flowing into the operating passages 155R. The passages 155R are now incommunication with the operating ports 165R, the cumulative overlaptherebetween defining an A4a orifice. The fluid flowing through theoperating ports 165R then flows into the annular chamber of 39c,combining with the fluid in the main fluid path. As was describedpreviously, the fluid returning from the steering cylinder 19 flowsthrough the annular chamber 41c, and from there, a portion flows throughthe remainder of the main fluid path, i.e., through the A5R orificedefined by the overlap between the operating ports 137R and the returnpassages 127R. The remainder of the fluid passes through an A5a orificedefined by the cumulative overlap between the operating ports 165L andthe tank passages 159R. All of this fluid then flows into the interiorof the spool 63, from where it flows to the system reservoir 13 in a"regulated" manner, to thus provide the pilot pressure required tocontrol the axial position of the sleeve 65.

MANUAL OVERRIDE

It is contemplated that the axial valving 49 may be actuated as part ofan automatic guidance system, which is in use at least part of the timein a vehicle in which an operator is still present. It is one importantaspect of the present invention for the operator to manually overridesuch an automatic guidance system in the event that, for example, theoperator suddenly becomes aware of the need to deviate from the nominalvehicle path determined by the automatic guidance system. Also, it isimportant for the vehicle operator to be able to manually steer thevehicle, rotating the steering wheel 17 to operate the meter 55 as ahand pump. In either case, it is necessary to be able to steer by meansof the rotary valving 47, without there being any flow through the axialvalving 49, despite the presence of a correction signal 31.

Referring now to FIG. 11, the spool and sleeve have been relativelydisplaced to a maximum rotary operating position in which the pressureports 131 are in fully open communication with the pressure passage125R, to achieve maximum flow through the rotary valving 47. With thespool and sleeve in this maximum rotary operating position, it may beseen in FIG. 11 that, within the axial valving 49, the pressure ports161 are now out of communication with the pressure passages 151R.Similarly, the operating ports 165R are out of communication with theoperating passages 155R, and the operating ports 165L are out ofcommunication with the tank passages 159R. Therefore, all of theorifices A1a, A4a, and A5a in the parallel fluid path are closed (equalto zero flow area), such that the only flow to the steering cylinder 19is that flowing through the rotary valving 47, i.e., through the mainfluid path.

As is now well known to those skilled in the art, when manuallysteering, using the meter 55 as a hand pump, it is essential that therenot be any "short-circuit" flow paths through the controller valving. Asis also well known, in a controller of the type to which the presentinvention relates, manual steering inherently occurs at maximum rotaryvalve deflection (i.e., the rotary position shown in FIG. 11).Therefore, by configuring the axial valving 49 as shown in FIG. 11, andclosing off the parallel fluid path, manual steering may beaccomplished. In addition, if the controller is operating as part of avehicle guidance system, and an axial sleeve position, as illustrated inFIG. 10 is commanded, if the operator becomes aware of a need to"override" the guidance system, the steering wheel 17 can be rotated inthe opposite direction to a maximum rotary operating position in theopposite direction. The spool moves "up" in FIG. 11 until the pressurepassage 125L is in open communication with the pressure ports 131. Atthe same time, various passages defined by the spool 63 will move "up"by the same amount, and the sleeve 65 may move substantially to theright in FIG. 11 (if the electronic controls are functioning properly),or may stay in the same axial position shown in FIG. 11 (if theelectronic controls are not functioning properly). In either case, eachof the pressure passages 151L will now be located "above" each of thecorresponding pressure ports 161, such that there is again no flowthrough the axial valving 49.

CONTROL LOGIC

Referring now to FIGS. 12 and 13, there will be described a preferredlogic control system for steering a vehicle, wherein the control systemmay advantageously include the fluid controller 15 which has previouslybeen described.

Although certain of the elements in the logic diagram of FIG. 12 are thesame as in the hydraulic schematic of FIG. 1, it is not thereby intendedto indicate any particular relationship between the schematics of FIG. 1and FIG. 12. The schematic of FIG. 1 was intended to illustrateprimarily the fluid controller 15, and its use in a simplified systemwherein the rate-select switch and controller 29 would receive the inputand output position signals 23 and 27, respectively, compare thosesignals, and generate a resulting correction signal 31. The correctionsignal 31 is transmitted to the pilot stage control 33 to therebymodulate the axial valving 49 in an attempt to "null" the correctionsignal 31, i.e., drive the output position signal toward the inputposition signal, until the correction signal 31 becomes zero.

By way of contrast, the control logic diagram of FIG. 12, in conjunctionwith the graph of FIG. 13, is intended to serve as a basis fordescribing various control functions and algorithms which are believedto be important features of an overall steering system of the type whichwould utilize the fluid controller 15.

Referring first to FIG. 12, the diagram will be described briefly, andthen the various control functions and algorithms will be describedsubsequently in greater detail. As in the schematic of FIG. 1, thesteering wheel 17 is provided with the steering wheel position sensor 21which generates the input position signal 23. The input position signal23 is transmitted to a look-up table 171, which also receives a vehiclestatus signal 173 from a rate-select switch 175. The rate-select switch175 may be used to provide the vehicle operator with an opportunity tomanually select between two (or more) different steering "gains", i.e.,different rates of change of steered wheel position for a given amountof rotation of the steering wheel 17.

Another input to the look-up table 171 is the output position signal 27which indicates actual, instantaneous steered wheel position. The outputfrom the look-up table 171 is a signal 177 which represents desiredsteered wheel angle (position). The desired steered wheel angle signal177 is transmitted to a summer 179, the other input to which is theoutput position signal 27. The output of the summer 179 is a positionerror signal 181 which, as is well known to those skilled in the art, isbasically the difference between the signals 177 and 27.

The desired steered wheel angle signal 177 is also transmitted to aderivative function circuit 183, the purpose of which is todifferentiate (take the derivative of) the signal 177. Therefor, as isalso well known to those skilled in the art, the output of the circuit183 is a signal 185 which represents the rate of change of the desiredsteered wheel angle. This rate of change signal 185 is used in severalof the control functions to be described subsequently. The rate ofchange signal 185 is transmitted to an integrator circuit 187, thefunction of which is to integrate the rate of change signal 185, i.e.,convert the rate of change signal 185 back into a signal representativeof position, and in this particular case, the desired position of thesteered wheels. Another input to the integrator circuit 187 is thestatus change signal 173, for reasons to be explained subsequently.

The rate of change signal 185 is also transmitted to a circuit 189 whichalso receives the position error signal 181, and has as its function, tomodify the gain of the integrator 187 by varying a gain signal 191. Thegain signal 191 is an additional input to the integrator circuit 187, asis the output position signal 27. The output of the integrator circuit187 is a command signal 193 which is transmitted to the positive inputof a P.I.D. control loop 195, the negative input to the P.I.D. 195 beingthe output position signal 27. As is well known to those skilled in theart, "P.I.D." means "Proportional-Integral-Differential". In the subjectembodiment, the P.I.D. control loop 195 would include at least the pilotstage control 33, the fluid controller 15, the steering cylinder 19, andthe steered wheel position sensor 25, such that the output of the P.I.D.control loop 195 is, as shown in FIG. 12, the output position signal 27.The function of the P.I.D. control loop 195 is to modulate the pilotstage control 33 in response to changes in the command signal 193, suchthat the output position signal 27 is driven toward the command signal193, i.e., the difference is "nulled out".

It is an important aspect of the control logic of the present inventionthat the command signal 193, which is representative of instantaneouslycommanded steered wheel position, is not merely equal to, or directlyproportional to, or representative of the position of the steering wheel17, which is the truest indication of the desired steered wheelposition. Instead, the command signal 193 may differ somewhat from thedesired steered wheel angle signal 177, at various times and undervarious circumstances, to permit the steering system to comply withseveral desired performance criteria. These criteria will be describedbriefly, and then there will be a subsequent, more detailed descriptionof the control functions and algorithms which enable the system tocomply with each of the particular criteria. The performance criteriainclude the following:

1. Upon vehicle start up, any "error" between actual steered wheelposition (actual position of the steering cylinder 19) and the positionof the steering wheel 17 should be corrected in a manner, and at a ratewhich is not noticeable to the vehicle operator;

2. Any change in the vehicle "status" should not result in an immediate,excessive correction of any error in the position of the steered wheels;

3. Whenever the steering wheel 17 is not being moved by the operator,there should be no movement of the steered wheels;

4. There should be movement of the steered wheels (movement of thesteering cylinder 19) only when the steering wheel 17 is being moved bythe operator;

5. Whenever the steering wheel is moved by the operator, the steeredwheels must move in the direction expected by the operator, and in theapproximate amount expected; and

6. During normal steering operations, and subject to the previouscriteria, there should be no deviation of the steered wheels from thedesired steered wheel angle (position).

START-UP

Referring now to FIG. 13, there is a graph of steering wheel and steeredwheel position versus absolute steering wheel displacement. In otherwords, the graph of FIG. 13 is based upon the assumption that thesteering "input device" to the fluid controller 15 is a steering wheelfor which there can be more than one desired steered wheel anglecorresponding to a particular steering wheel position. In the subjectembodiment, by way of example only, the system of the present inventionwill be described in connection with a vehicle in which the steeringwheel 17 can be rotated six turns lock-to-lock. In other words, from theneutral, centered position, the steering wheel 17 may be rotated threeturns counterclockwise before engaging a stop, or may be rotated threeturns clockwise before engaging a stop.

Superimposed on the graph of steering wheel position (input positionsignal 23) is a graph of steered wheel position (output position signal27). Thus, it may be seen that, by way of example only, each rotation ofthe steering wheel 17 should turn the steered wheels 15 degrees, suchthat rotation of the steering wheel against either stop should result inturning of the steered wheels 45 degrees in that particular direction.Upon engine or vehicle start up, several different criteria and controlfunctions come into play.

Whenever the vehicle engine starts, or there is some other change in thevehicle status, such as the rate select switch 175 being moved to adifferent "Rate" position, an appropriate vehicle status signal 173 istransmitted to the look up table 171, and to the integrator circuit 187.Whenever the status signal 173 indicates a status change, the integratorcircuit 187 is reset, and the command signal 193 is initialized to beequal to the steered wheel position signal 27 so that, initially, nocorrection of steered wheel position is made. This satisfies theforegoing criteria numbers one and two.

After the instantaneous initialization, correction can occur in view ofthe possibility of misalignment of the steering and steered wheels.Referring again to both FIGS. 12 and 13, upon start-up, the steeringwheel position is sensed in the normal manner, and the instantaneoussteering wheel position 23 is transmitted to the look up table 171. Ifthe instantaneous steering wheel position is represented by the linelabeled "I" in the graph of FIG. 13, it may be seen that that Particularsteering wheel position could correspond to any one of six differentsteered wheel positions on the graph 27. Note the six vertical linesextending to the graph 27 from the six intersections of the line I andthe graph 23. It should also be noted in FIG. 13 that there is a dashedline including a point "27i", representing initial steered wheelposition.

The initial steered wheel position signal 27i is then comparedmathematically with each of the theoretically possible steered wheelposition signal 27-1 through 27-6, to see which pair of signals has thesmallest absolute difference. In the example presented in FIG. 13, itmay be seen that the least absolute difference exists between theinitial signal 27i and the signal 27-5, thus indicating that the steeredwheel position signal 27 should be equal to the signal 27-5, above thefifth ramp of the steering wheel position signal 23.

In the example shown in FIG. 13, the initial steered wheel positionsignal 27i is disposed to the right of the signal 27-5, which means thatthe actual steered wheel position (signal 27i) is "leading" the desiredsteered wheel position (signal 27-5), assuming steering wheel rotationin the clockwise direction. As a result, in order to achieve correctionof the error between the signal 27i and 27-5, a gain factor of less than1.0 is applied by the circuit 189 in generating the gain signal 191which determines the gain of the integrator circuit 187. By "leading",it is meant that the steered wheels are displaced further in thedirection of steering than is the steering wheel. By way of contrast, ifthe initial steered wheel position signal 27i were disposed to the leftof the desired position signal 27-5, still assuming steering wheelrotation in the clockwise direction, such would be an indication thatthe steered wheel position is "lagging" the steering wheel position, inwhich case the circuit 189 would generate a gain signal 191 greater than1.0, to enable the actual steered wheel position to catch up to thesteering wheel position. By way of example only, in the subjectembodiment, when the steered wheel position is leading, a gain of 0.8 isapplied, whereas when the steered wheel position is lagging, a gain of1.2 is applied. As will be apparent to those skilled in the art, theamount the gain signal 191 deviates from 1.0 could be a function of therelative magnitude of the position error signal 181.

NORMAL STEERING

During normal steering, after the initial start-up procedure describedabove, the P.I.D. control loop 195 is constantly being "driven" bycontrolling the flow of fluid to the steering cylinder 19, to "null out"any error between the steered wheel position signal 27 and the commandsignal 193.

During normal steering, or more precisely, during normal vehicleoperation, there are two possible operating conditions of the steeringwheel 17. Either the wheel is being rotated at some rate, or the wheelis stationary.

In accordance with criteria number 3, if the steering wheel 17 isstationary, there should be no movement of the steered wheels. In otherwords, there should be no "drift" of the steered wheels. It is partlybecause of the two performance criteria involving the presence orabsence of the steering wheel movement that the derivative functioncircuit 183 is included in the logic control system of FIG. 12. When thesteering wheel is stationary, the rate of change signal 185 is 0 (thederivative of a constant equals 0). When the rate of change signal 185is 0, the function of the integrator circuit 187 is to generate acommand signal 193 which will drive the P.I.D. control loop 195 tomaintain the steered wheel position signal 27 constant. It should beclearly understood that requiring that there be no movement of thesteered wheels, when there is no movement of the steering wheel 17, doesnot mean that there will not be any flow of fluid to the steeringcylinder 19. In a typical full-fluid link hydrostatic power steeringsystem, fluid leakage within the system typically results in movement ofthe steered wheels even in the absence of movement of the steeringwheel. Therefor, it is one feature of the logic control system of thepresent invention to generate an appropriate command signal 193 suchthat a small amount of fluid, if needed, will be communicated to thesteering cylinder 19 to maintain a fixed steered wheel position as longas the steering wheel 17 is stationary.

When the vehicle operator begins to rotate the steering wheel 17, thesteered wheel signal 177 changes, and the derivative function circuit183 produces a rate of change signal 185 which is not equal to zero.Only when the integrator circuit 187 is receiving a rate of changesignal 185 which is not equal to zero will it generate a command signal193 which is capable of permitting a change in the steered wheelposition, and thus, in the position signal 27. Thus, performancecriteria numbers 3 and 4 above are satisfied.

While the vehicle operator is rotating the steering wheel, the sign(positive or negative) of the rate of change signal 185 indicates thedirection of rotation of the steering wheel. The integrator circuit 187receives the signal 185 and senses the sign of the signal 185, andinsures that an appropriate command signal 193 is generated such thatthe direction of movement of the cylinder 19 corresponds to thedirection of rotation of the steering wheel. Thus, performance criterianumber 5 above is satisfied.

The invention has been described in great detail in the foregoingspecification, and it is believed that various alterations andmodifications of the invention will become apparent to those skilled inthe art from a reading and understanding of the specification. It isintended that all such alterations and modifications are included in theinvention, insofar as they come within the scope of the appended claims.

I claim:
 1. A method of controlling the flow of fluid from a source ofpressurized fluid through a fluid controller in response to the positionand movement of an input device, to cause the position of a steeringcylinder to conform to the position of said input device, said methodcomprising the steps of:(a) providing said fluid controller with mainvalving operable to define a main fluid path, and control fluid flowtherethrough, in response to the movement of said input device, andauxiliary valving operable to define an auxiliary fluid path, andcontrol the fluid flow therethrough, in response to changes in a commandsignal; (b) sensing the position of said input device, and generating aninput position signal; (c) sensing the position of said steeringcylinder, and generating an output position signal; (d) comparing saidoutput position signal to said input position signal, and generatingsaid command signal; (e) transmitting a signal representative of saidcommand signal to said auxiliary valving and modulating said auxiliaryvalve to drive said output position signal toward said input positionsignal, and (f) sensing a vehicle status change, generating a statuschange signal in response to the sensing of a vehicle status change, andsetting said command signal equal to said output position signal inresponse to the presence of said status change signal.
 2. A method ofcontrolling the flow of fluid from a source of pressurized fluid througha fluid controller in response to the position and movement of an inputdevice, to cause the position of a steering cylinder to conform to theposition of said input device, said method comprising the steps of:(a)providing said fluid controller with valving which defines:(i) mainvalving operable to define a main fluid path, and control fluid flowtherethrough, in response to the movement of said input device, and (ii)auxiliary valving operable to define an auxiliary fluid path, saidauxiliary fluid path having upstream and downstream locations incommunication with said main fluid path, said upstream and downstreamlocations being disposed within said controller, said auxiliary valvingbeing operable to control the fluid flow through said auxiliary fluidpath, in response to changes in a command signal; (b) sensing theposition of said input device, and generating an input position signal;(c) sensing the position of said steering cylinder, and generating anoutput position signal; (d) comparing said output position signal tosaid input position signal, and generating said command signal; (e)transmitting a signal representative of said command signal to saidauxiliary valving and modulating said auxiliary valve to drive saidoutput position signal toward said input position signal; and (f)sensing a vehicle status change, generating a status change signal inresponse to the sensing of a vehicle status change, and setting saidcommand signal equal to said output position signal in response to thepresence of said status change signal.
 3. A method of controlling theflow of fluid from a source of pressurized fluid through a fluidcontroller in response to the position and movement of an input device,to cause the position of a steering cylinder to conform to the positionof said input device, said method comprising the steps of:(a) providingsaid fluid controller with valving which defines:(i) main valvingoperable to define a main fluid path and control fluid flowtherethrough, in response to the movement of said input device, and (ii)auxiliary valving operable to define an auxiliary fluid path, saidauxiliary fluid path having upstream and downstream locations incommunication with said main fluid path, said upstream and downstreamlocations being disposed within said controller, said auxiliary valvingbeing operable to control the fluid flow through said auxiliary fluidpath, in response to changes in a command signal; (b) sensing theposition of said input device, and generating an input position signal;(c) sensing the position of said steering cylinder, and generating anoutput position signal; (d) comparing said output position signal tosaid input position signal, and generating said command signal; (e)transmitting a signal representative of said command signal to saidauxiliary valving and modulating said auxiliary valve to drive saidoutput position signal toward said input position signal; and (f)providing an input device which is rotatable by an amount greater thanone rotation from a counter-clockwise stop position to a clockwise stopposition, and upon vehicle start-up, determining a plurality of desiredpositions of said steering cylinder corresponding to said input positionsignal; comparing said output position signal to each of said pluralityof desired positions and selecting the desired position having the leastabsolute difference, compared to said output position signal; and, if anerror exists between said output position signal and said input positionsignal, modifying said command signal by a factor greater than 1.0 ifsaid steering cylinder is lagging said input device, or by a factor lessthan 1.0 if said steering cylinder is leading said input device.
 4. Amethod of controlling the flow of fluid from a source of pressurizedfluid through a fluid controller in response to the position andmovement of an input device, to cause the position of a steeringcylinder to conform to the position of said input device, said methodcomprising the steps of:(a) providing said fluid controller with valvingwhich defines:(i) main valving operable to define a main fluid path, andcontrol fluid flow therethrough, in response to the movement of saidinput device, and (ii) auxiliary valving operable to define an auxiliaryfluid path, said auxiliary fluid path having upstream and downstreamlocations in communication with said main fluid path, said upstream anddownstream locations being disposed within said controller, saidauxiliary valving being operable to control the fluid flow through saidauxiliary fluid path, in response to changes in a command signal; (b)sensing the position of said input device, and generating an inputposition signal, generating a derivative signal representative of therate of change of said input position signal, and generating saidcommand signal operable to prevent movement of said steering cylinderwhenever said derivative signal indicates absence of movement of saidinput device; (c) sensing the position of said steering cylinder, andgenerating an output position signal; (d) comparing said output positionsignal to said input position signal, and generating said commandsignal; and (e) transmitting a signal representative of said commandsignal to said auxiliary valving and modulating said auxiliary valve todrive said output position signal toward said input position signal. 5.A method of controlling the flow of fluid from a source of pressurizedfluid through a fluid controller in response to the position andmovement of an input device, to cause the position of a steeringcylinder to conform to the position of said input device, said methodcomprising the steps of:(a) providing said fluid controller with valvingwhich defines:(i) main valving operable to define a main fluid path, andcontrol fluid flow therethrough, in response to the movement of saidinput device, and (ii) auxiliary valving operable to define an auxiliaryfluid path, said auxiliary fluid path having upstream and downstreamlocations in communication with said main fluid path, said upstream anddownstream locations being disposed within said controller, saidauxiliary valving being operable to control the fluid flow through saidauxiliary fluid path, in response to changes in a command signal; (b)sensing the position of said input device, and generating an inputposition signal, and generating a derivative signal representative ofthe rate of change of said input position signal, and direction ofchange of said input position signal, and generating said command signaloperable to cause movement of said steering cylinder only while saidderivative signal indicates movement of said input device, and only inthe direction corresponding to the direction of movement of said inputdevice; (c) sensing the position of said steering cylinder, andgenerating an output position signal; (d) comparing said output positionsignal to said input position signal, and generating said commandsignal; and (e) transmitting a signal representative of said commandsignal to said auxiliary valving and modulating said auxiliary valve todrive said output position signal toward said input position signal.